Make a version-less RHEL element to handle both '7' and '8' DIB_RELEASE.
The element usage should align with other elements which operate in the
same way such as the Fedora element.
Additionally, this patch adds support for RHEL8 that operates with
Python 3.
As of now, users of diskimage-builder will still be able to use the
'rhel7' element, or migrate to 'rhel' and specify their respective
DIB_RELEASE value.
* mount the xfs file-system for extraction as read-only. vaguely
based on explaination in [1] and the fact we only read the image
data into a tar, so can ignore this.
XFS (dm-1): Superblock has unknown read-only compatible features (0x4) enabled.
* Use the redhat system python as the dib-python version. dib was
ahead of it's time making an abstracted python interpreter for
system work ;) the system python should work for running the various
dib element scripts.
[1] https://unix.stackexchange.com/questions/247550/unmountable-xfs-filesystem
Redhat-Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1700253
Co-Authored-By: Ian Wienand <iwienand@redhat.com>
Change-Id: I90540675c70bb475d9db2ae24f81c648a31f3f95
